Project Summary The human intestinal tract supports a complex microbial environment consisting of bacterial (or microbiota) and fungal (or mycobiota) constituents. Although the role of each of these organisms in eliciting immune activation and inflammation in the gut as well as their ability to initiate systemic infection has begun to be investigated and appreciated by the larger scientific community, their interspecies interactions within the context of the gastrointestinal tract remains an underrepresented area of research. The chemical basis for such interactions, critical for the rational design of treatments in gastrointestinal infection and disease, remain completely uncharted territory in the literature. This lies in stark contrast to the vibrant fields of terrestrial and marine secondary metabolite structural determination and bioactivity, where a seemingly endless stream of biosynthesized natural products and effector pathways have been elucidated between fungi and bacteria. Our preliminary ribosomal 16S sequencing data show that specific communities of anaerobic bacteria are drastically increased in a reproducible way with a variety of antifungal medications. This analysis indicates that bacteria and fungi may occupy a similar, competitive ecological niche within the gut ecosystem. To illuminate the potential for bacterial metabolites to influence the mycobiota, thereby establishing a competitive advantage, we developed a library of known gut metabolites and screened for antifungal activity at physiologically relevant conditions in vitro. This resulted in the identification of two metabolites of bacterial origin with activity towards opportunistic members of the mycobiota. We therefore hypothesize that opportunistic pathogenic fungi in the gut are held in check by bacterial metabolite production and that this mechanism is stimulated by intestinal fungi to impact infectious states. In addition to revealing novel mechanisms of fungal-bacterial interaction at an unprecedented small molecule level, the results of this proposed investigation will illuminate potential new strategies for targeting of fungal pathogens.